The present invention relates to a process for oxidizing silicon monoxide, silicon vapor, and hypostoichiometric silicon dioxide in a crystal pulling apparatus. More particularly, the present invention relates to a continuous oxidation process wherein an oxygen-containing gas is continuously injected into a crystal pulling apparatus to continuously oxidize silicon monoxide, silicon vapor, and hypostoichiometric silicon dioxide as it is produced.
Single crystal silicon which is the starting material for most processes for the fabrication of semiconductor electronic components is commonly prepared with the so-called Czochralski process. In this process, a crystal pulling apparatus purged with a continuous stream of argon is utilized wherein polycrystalline silicon ("polysilicon") is charged to a quartz crucible, the polysilicon is melted, a seed crystal is immersed into the molten silicon and a single crystal silicon ingot is grown by slow extraction.
During the crystal growth process, substantial amounts of silicon containing compounds such as gaseous silicon monoxide (SiO(g)) and silicon vapor (Si(g)) are produced and released into the atmosphere in the apparatus. SiO(g) is evaporated from the melt due to the enriching with oxygen through the dissolution of the crucible by the silicon melt, as well as the interaction of the crucible with the graphite susceptor supports. Additionally, the vapor pressure of the Si(g) over the silicon liquid at the silicon melting point is about 0.5 Torr which may account for a moderate quantity of Si(g) being produced during ingot growth. Because SiO(g) is extremely unstable, it readily reacts with other molecules of Sio(g) produced from the crucible dissolution to produce silicon dioxide (solid) (SiO.sub.2 (s)) and silicon (solid) (Si(s)). However, because the amount of available oxygen within the crystal pulling apparatus is limited, a substantial amount of the SiO.sub.2 (s) which is formed is oxygen deficient, or hypostoichiometric. Also, the Si(g) evaporated from the melt which may condense on the hot zone exhaust system parts is very reactive with oxygen.
The mixture of hypostoichiometric SiO.sub.2 (s) and condensed Si(g) produced as a by-product in the crystal pulling apparatus is a major problem in the crystal pulling industry as it tends to stick and become attached to many parts of the pulling apparatus and exhaust system. When the apparatus and exhaust system are opened to the atmosphere after the ingot is grown, and the hypostoichiometric SiO.sub.2 (s) is mixed with large amounts of oxygen, it can smolder, burn and release substantial amounts of heat which can cause serious damage to internal parts of the apparatus as well as the exhaust system. Furthermore, if sufficient amounts of hypostoichiometric SiO.sub.2 (s) are present in a dust form, an explosion and resulting rapid over pressurization of the pulling apparatus can occur and cause substantial damage to the apparatus and workers.
Conventionally, the dangers of rapid over pressurization of the apparatus and burning of parts due to the reaction of hypostoichiometric SiO.sub.2 (s) and oxygen has been controlled through the use of a post-run oxidation step which consists of backfilling the evacuated crystal puller with an oxygen-containing gas such as air at a slow rate over a period of up to about 24 hours. The oxygen in the air slowly reacts with and oxidizes the hypostoichiometric SiO.sub.2 (s) to produce SiO.sub.2 (s) containing sufficient oxygen and reduces the risk of over pressurization. By adding time to the process cycle, the throughput of the pulling apparatus is significantly reduced. Additionally, because up to about 95% of the hypostoichiometric SiO.sub.2 (s) accumulation is on water cooled surfaces or exhaust piping several feet from the hot zone and hence near room temperature, the oxidation rate of the accumulated oxide is much less efficient than desired and some risk to equipment and workers may still exist.
Therefore, a need exists in the semiconductor industry for an efficient process that can substantially eliminate the dangers of hypostoichiometric SiO.sub.2 (s) without decreasing the throughput of the crystal pulling apparatus.